RFID (radio frequency identification) systems are widely used in the automotive industry and other industries such as farm animal identification, building access control, and meter reading. A primary application in the automotive industry is the portion of the anti-theft system that prevents a vehicle from being started without a key or a remote control device that has been electronically paired to a specific vehicle. RFID systems often include low power radio transponders. The transponder receives a radio signal from a base unit (also known as a “reader”). The reader may be built into a vehicle or other type of system. The transponder can be powered either by a power supply, a battery, or by energy harvested from the radio frequency (RF) field that is produced by the reader. Some transponders are primarily powered by a battery with the capability of being powered by the RF field when the battery cannot provide sufficient energy or when the battery is absent.
Upon receiving a specific RF signal from the reader, the transponder responds and transmits an RF signal with specific characteristics that can be detected by the reader. The RF signals generated by the reader and transponder are typically modulated with data. The data transmitted by the reader and transponder is system dependent. Data transmitted by the transponder may include a transponder identification code, the reading of a water meter, or data from a vehicle key to be validated by the anti-theft system. A transponder in a vehicle key or in the vehicle itself can be considered an “immobilizer” because, if the reader fails to receive the correct response from the transponder in the key or a key fob, the vehicle or equipment is disabled and may not start.
Efficient use of power by the transponder is desired in order to maximize battery life. Also, efficient use of power is necessary when operating with the small amount of energy that can be harvested from an RF field when the battery is absent or discharged.